Fibre-optics cables are in wide-spread use for carrying telecommunications signals. A number of methods commonly used to create fibre optics telecommunications networks involve first establishing a network of ducts between required locations and then introducing the fibre optics cables into the network of ducts using techniques such as the “blown fibre” and “blown cable” methods. These methods involve driving the fibre optic cable along the duct with compressed air, for example as described in EP 0108590 (British Telecommunications). In order for such methods to work efficiently, it is desirable that the network of ducts should be sufficiently gas-tight to enable maintenance of the gas pressures needed to convey the cables along the ducts. For this reason, open ends of ducts at terminal locations on the network are typically fitted with plugs (end-stops) that have seals to prevent or limit the escape of gas. The end-stops also serve to prevent foreign objects or materials (e.g. dust and small creatures) from passing into the network.